This invention relates to luminescent materials and methods of making luminescent materials. More particularly, it is concerned with yellow-emitting tin and manganese coactivated calcium fluorophosphate phosphors and with methods of making such phosphors having improved brightness.
It is known to utilize halophosphate phosphors in fluorescent lamps generating one of a number of standard white spectral distributions. Manganese activated halophosphate phosphors are well known in the art for this purpose. Normally, however, these phosphors are not ideally suited for low-pressure mercury-vapor fluorescent lamp applications when activated with manganese alone. Manganese does not appreciably absorb short-wavelength ultraviolet light in the 254 nanometer region. Hence a sensitizer ion, capable of absorbing short-wavelength ultraviolet and of subsequently transferring the absorbed energy to manganese, is required to make such phosphors efficient for this application. In the family of commercially available calcium halophosphate phosphors activated with manganese, antimony is typically employed as the sensitizer ion.
It has been shown, however, that other ions can be used to sensitize manganese in halophosphate phosphors. British patent specification No. 877,536, for example, discloses calcium halophosphate phosphors coactivated with copper and manganese or with tin and manganese. The method for the production of the phosphors disclosed there involves repeated firing of the phosphor precursor blend in an applied weakly reducing atmosphere of hydrogen and nitrogen to optimize brightness output of the phosphor.